The present invention pertains to a system and method for efficiently cancelling interference in a radio communication system using a directional antenna and one or more search beams.
FIG. 1 illustrates a conventional cellular radio communication system 100. The radio communication system 100 includes a plurality of radio base stations 170a-n connected to a plurality of corresponding antennas 130a-n. The radio base stations 170a-n in conjunction with the antennas 130a-n communicate with a plurality of mobile terminals (e.g. terminals 120a, 120b and 120m) within a plurality of cells 10a-n. Communication from a base station to a mobile terminal is referred to as the downlink, whereas communication from a mobile terminal to the base station is referred to as the uplink.
The base stations are connected to a mobile telephone switching office (MSC) 150. Among other tasks, the MSC coordinates the activities of the base stations, such as during the handoff of a mobile terminal from one cell to another. The MSC, in turn, can be connected to a public switched telephone network 160, which services various communication devices 180a, 180b and 180c. 
A common problem that occurs in a cellular radio communication system is the loss of information in the uplink and downlink signals as a result of multi-path fading, which results when the transmitted signal travels along several paths between the base station and the intended receiver. When the path lengths between the base station and the mobile terminal are relatively small, the multiple signal images arrive at almost the same time. The images add either constructively or destructively, giving rise to fading, which typically has a Rayleigh distribution. When the path lengths are relatively large, the transmission medium is considered time dispersive, and the added images can be viewed as echoes of the transmitted signal, giving rise to intersymbol interference (ISI).
Fading can be mitigated by using multiple receive antennas and employing some form of diversity combining, such as selective combing, equal gain combining, or maximal-ratio combining. Diversity takes advantage of the fact that the fading on the different antennas is not the same, so that when one antenna has a faded signal, chances are the other antenna does not. ISI from multi-path time dispersion can be mitigated by some form of equalization, such as linear equalization, decision feedback equalization, or maximum likelihood sequence estimation (MLSE).
Interference can also degrade the signals transmitted between a base station and mobile terminals. For instance, a desired communication channel between a base station and a mobile terminal in a given cell can be degraded by the transmissions of other mobile terminals within the given cell or within neighboring cells. Other base stations or RF-propagating entities operating in the same frequency band can also create interference (through xe2x80x9cco-channelxe2x80x9d or xe2x80x9cadjacent channelxe2x80x9d interference).
Frequency re-use can be used to mitigate interference by locating interfering cells as far from each other as possible. Power control can also be used to reduce the interference by ensuring that transmitters communicate at minimal effective levels of power. Such power control techniques are especially prevalent in code-division multiple access systems, due to the reception of information in a single communication channel at each base station.
Interference can be reduced still further by using a plurality of directional antennas to communicate with mobile terminals within a cell. The directional antennas (also known as xe2x80x9csector antennasxe2x80x9d) transmit and receive energy within a limited geographic region, and thereby reduce the interference experienced by those radio units outside such geographic region. Typically, radio communication cells are partitioned into three 120xc2x0 sectors serviced by three sector antennas, or six 60xc2x0 sectors serviced by six sector antennas. Even smaller antenna sectors can be achieved using a fixed-beam phased array antenna, which transmits and receives signals using a plurality of relatively narrow beams. FIG. 2, for instance, illustrates such an exemplary radio communication system 200 including a radio base station 220 employing a fixed-beam phased array (not shown). The phased array generates a plurality of fixed narrow beams (B1, B2, B3, B4, etc.) which radially extend from the base station 220. Preferably, the beams overlap to create a contiguous coverage area to service a radio communication cell. Although not shown, the phased array can actually consist of three phased array sector antennas, each of which communicates with a 120xc2x0 swath extending from the base station 220.
FIG. 2 shows a mobile terminal 210 located within the coverage of one of the beams, B1. Communication proceeds between the base station 220 and this mobile terminal 210 using the beam B1, or perhaps, in addition, one or more adjacent beams. The reader will appreciate that modern radio communication environments typically include many more mobile terminals within cells. Nevertheless, even when there are plural mobile terminals within a cell, a subset of the beams may not include any mobile terminal stations within their coverage. Hence, in conventional fixed-beam phased array systems, these beams remain essentially idle until a mobile terminal enters their assigned geographic region. Such idle beams propagate needless energy into the cell, and thus can contribute to the net interference experienced by radio units within the cell as well as other cells (particularly neighboring cells). These beams also add to the processing and power load imposed on the base station 220.
These concerns are partly ameliorated though the use of a variation of the above-discussed system, referred to as xe2x80x9cadaptivexe2x80x9d phased arrays. Such arrays allow for the selective transmission and reception of signals in a particular direction. For instance, as shown in FIG. 3, an array 300 can be used to receive a signal transmitted at an angle xcexa (with respect to the normal of the array) from a target mobile terminal 380, and can simultaneously cancel the unwanted signals transmitted by another mobile terminal 370. This is accomplished by selecting weights (w1, w2, . . . wn) applied to each signal path (r1, r2, . . . r3) from the phase array antenna 300 so as to increase the sensitivity of the array in certain angular directions and reduce the sensitivity of the array in other directions (such as by steering a null toward an interference source). The desired weighting is selected by iteratively changing the weights through a feedback loop comprising beamforming unit 340, summer 330 and controller 320. The feedback loop functions to maximize signal-to-interference ratio at the output xe2x80x9cxxe2x80x9d of the beamforming unit. Application of an adaptive phased array antenna to the radio communication system shown in FIG. 1 would result in the generation of a single beam (or small subset of beams) generally oriented in the direction of the single mobile terminal 210. Such a system offers a substantial reduction in interference. For example, as disclosed in xe2x80x9cApplications of CDMA in Wireless/Personal Communicationsxe2x80x9d by Garg et al., Prentice Hall, 1997, an idealized eight-beam antenna could provide a threefold increase in network capacity when compared with existing schemes such as cell splitting (pp. 332-334). Interested readers are referred to the following documents for further details regarding adaptive phased arrays as well as information regarding adaptive diversity arrays: xe2x80x9cAdaptive Arrays and MLSE Equalizationxe2x80x9d by G. E. Bottomley et al., Proc. VTC ""95, Chicago, Ill., July 1995, pp. 50-54; xe2x80x9cSignal Acquisition and Tracking with Adaptive Arrays in the Digital Mobile Radio System IS-54 with Flat Fadingxe2x80x9d by J. H. Winters, IEEE Transactions on Vehicular Technology, Vol. 42, No. 4, November 1993; xe2x80x9cAdaptive Array Methods for Mobile Communicationxe2x80x9d by S. Simanapalli, Proc. 44th IEEE Veh. Technol. Conf., Stockholm, Sweden, Jun. 7-10, 1994, pp. 1503-1506; and published patent application No. WO 94/09568 to P. H. Swett et al., published 1994.
The presence and location of mobile terminals in both the fixed and adaptive beamforming cellular radio communication systems can be determined by measuring the signal strength in the uplink direction on each beam. The beam direction yielding the strongest received signal would indicate the probable location of the desired mobile. This technique, however, is not fully satisfactory. Often, for instance, due to multi-path fading, the beam yielding the strongest signal may not precisely correspond to the direction of the mobile user. Even if the strongest beam does correspond to the direction of the mobile user, the presence of multi-path fading and interference on other beams may degrade the quality of communication between the base station and the mobile terminal using the strongest beam. Furthermore, successively examining each beam generated by the phased array to locate a mobile user requires a significant amount of processing overhead. This overhead can reduce the response time of the base station.
It is therefore an exemplary objective of the present invention to provide a method and system for conducting communication between two radio units which does not suffer from the above-described drawbacks.
According to a first exemplary aspect of the present invention, the above objective is achieved through a base station using a fixed-beam phased array antenna which employs a first set of beams and associated hardware for conducting communication with a set of mobile terminals within a radio communication cell, and employs a second set of beams and associated hardware for searching the radio communication cell for the presence of candidate beams which should be added to the first set of beams. In the following discussion the beams in the first set are referred to as xe2x80x9cdecoding beamsxe2x80x9d, while beams in the second set are denoted xe2x80x9csearcher beamsxe2x80x9d.
According to a second exemplary aspect of the present invention, subsets of the decoding beams are processed by an equalizer, and are preferably processed by the interference-rejection-combining receiver disclosed in commonly assigned U.S. application Ser. No. 07/284,775, filed on Feb. 8, 1994. This receiver combines signals received from each subset of decoder beams and separates the wanted signals from the unwanted (interfering) signals.
According to a third exemplary aspect of the present invention, the base station determines the xe2x80x9cmembershipxe2x80x9d of each subset of decoder beams by successively examining each beam within the searcher set of beams. Those searcher beams (or combination of searcher beams) which meet prescribed criteria are selected and allocated to the task of processing a call from a mobile terminal. The beam is xe2x80x9callocatedxe2x80x9d in the sense that its associated hardware (e.g. comprising filters, downconverters, etc.) is allocated to the task of processing the call.
According to a fourth exemplary aspect of the present invention, the searcher beams and their associated hardware are used to determine the presence of new mobile terminals within a cell, including those terminals which have entered the cell from a neighboring cell, and those terminals which have initiated calls within the cell. The searcher beams and associated hardware are also used to determine the departure of terminals within a cell, including those terminals which have physically left the cell and those terminals which have simply terminated calls within the cell.
According to a fifth exemplary aspect of the present invention, the receiver/equalizer also interrogates the allocated decoder beams to determine whether these beams continue to possess signal characteristics which warrant their membership in the decoder set of beams. If a decoder beam no longer meets the prescribed criteria, it is returned to the searcher pool of beams. Thus, the allocation of beams (and associated hardware) to the decoder beam set and the searcher beam set is a dynamic process which takes into account all activity within the cell and outside the cell which affects the interference profile within the cell. According to one exemplary criterion, searcher beams are converted into decoder beams when they contain signal strength and/or signal quality characteristics above a prescribed threshold.
According to a sixth exemplary aspect of the invention, instead of a fixed beamforming phased array antenna, the base station can employ an adaptive phased array antenna. In this embodiment, a single searcher beam can be used to interrogate the cell to recruit candidates for inclusion in the decoder set of beams and to determine out-of-date members in the decoder set of beams. By appropriate weighting of the phased array, the base station steers the single searcher beam over a prescribed swath of geographic coverage. In alternative exemplary embodiments, more than one searcher beam can be employed.
According to a seventh exemplary aspect of the invention, the above-described cellular techniques can be employed in the indoor cellular environment. In this case, the radio heads are divided into a first set of decoder radio heads which are allocated to the task of processing calls, and a second set of radio heads which are allocated to the task of ensuring that the decoder set of radio heads remains optimal or near-optimal. Again, the signals provided by the decoder set of radio heads are processed using a receiver, preferably using the interference-rejection-combining receiver mentioned above.
According to an eighth exemplary aspect of the invention, the above-described cellular techniques can be used by a base station to locate one or more orbiting satellites by employing a decoder set of beams which are assigned for communicating with one or more satellites and a second searcher set of beams for canvassing a sector of space to ensure that the decoder beams remain optimal or near-optimal by recruiting searcher beams which meet prescribed criteria for inclusion in the decoder set of beams and rejecting decoder beams which fail to meet the prescribed criteria.